FAQ: How Exactly Do HGV Trailer Brakes Work?

faqs -> braking

1. Air Supply and Brake Circuits

HGV trailers in the UK and EU use a pneumatic (air) braking system, where compressed air is supplied from the tractor unit via two airlines:

Red Line (Emergency Line)

• Recharges the trailer’s air system and maintains pressure.

Yellow Line (Service Line)

• Transfers the driver’s braking demand to the trailer by varying air pressure.
• When the driver presses the footbrake, this line modulates the braking force.

2. Key Components of the Trailer Braking System

HGV trailers use a dual-chamber braking system, which consists of both the service brake system (yellow airline) and the spring brake system (controlled via the red airline and park brake).

1. Dual-Chamber Brake Actuators (Spring Brake & Service Brake)

Service Brake Chamber (Controlled by the Yellow Line)

• Applies the brakes when the driver presses the footbrake.
• Air pressure from the yellow service line moves a diaphragm inside the actuator, applying the brakes in proportion to pedal force.
• When the driver releases the foot brake pedal, the air is vented, and the brakes release.

Spring Brake Chamber (Fail-Safe Parking Brake, Controlled by Air Pressure)

• The spring brakes (parking brake) are held off by compressed air when the trailer is in normal operation (minimum ~3.5 BAR).
• When the trailer park brake valve is applied, air is vented from this chamber, allowing the powerful mechanical springs to apply the brakes.

If the red airline is disconnected:

• Older trailers (non-EBS): The service brakes apply immediately as part of the emergency braking system.
• Modern trailers with EBS: The spring brake chambers exhaust, allowing the powerful springs to automatically engage the brakes.

2. Air Reservoir / Tanks

• Stores compressed air from the red airline to maintain primary and auxiliary system pressure.
• EU Regulation 13 requires pressure to range between 6.5 and 8.5 BAR.

3. Load Sensing Valve (LSV)

• Adjusts braking force relative to trailer load to prevent wheel lock-up when empty.

4. Relay Valve

• Speeds up brake application by quickly directing stored air to the brake chambers.

5. Brake Drums or Discs

• Drum brakes (older, cost-effective, robust).
• Disc brakes (better stopping power, lower fade).

6. ABS (Anti-lock Braking System) and EBS (Electronic Braking System)

• ABS prevents wheel lock-up under heavy braking.
• EBS (on modern trailers) electronically controls braking for improved efficiency and stability.

7. Shunt Valve

• Used by shunters to manually release the spring brakes without needing to connect the red air supply.
• This allows short-distance movements within a yard, avoiding the need for full coupling.

8. Twin Chamber Alternative: Piston Brake Actuators

While most HGV trailers use traditional twin-chamber diaphragm brake actuators, some heavy-duty applications use piston-type brake actuators. These units replace the rubber diaphragm with a rigid piston, offering greater durability and more precise braking control. However, piston brakes are less common in standard UK/EU trailer braking systems and are typically found in specialist vehicles.

Conclusion

HGV trailer braking systems rely on a combination of pneumatic control, mechanical force, and modern electronics to ensure safety and efficiency.

While older trailers apply service brakes when the red airline is disconnected, modern EBS trailers instead exhaust the spring brake chambers, allowing the spring brakes to engage automatically.

Drivers must always ensure the trailer’s parking spring brakes are properly engaged before coupling or uncoupling.

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